My actual MTTD data points start at different times depending on whether there is a forecast being displayed. Why?

MTTD is not truly relevant until the entire system is integrated and functioning as a complete entity.  For this reason, MTTD values are not shown until Systems Integration Test (which occurs at about 71% of phase 3).   

Because we do not know the actual phase 3 time until the system is completed, we show actual MTTD data points starting at the same time as the plan unless there is a forecast present. Planned MTTD data points will always begin at about 71% of the planned phase 3 time. 

Once a forecast is present, the actual/forecast MTTD data points will be shown starting at 71% of the forecasted phase 3 time.